Polyurethanes have a very wide range of applications in surface coatings, protective flooring products, adhesives, sealants and the like to provide protective surfaces with high resistance to aggressive chemicals, physical damage and abrasion. They exhibit very good adhesion to most substrates and can provide long term protection against corrosion and erosion in the most severe of environments. They are generally made by reacting together the components of a two-component composition, one of which includes a polyisocyanate while the other includes a component having two or more hydroxyl groups to react with the polyisocyanate.
It is to be understood that by "a two-component composition" we mean a composition comprising two essential components. Such a composition may additionally comprise one or more other optional components.
Although it is possible to manufacture single component coating systems containing available isocyanate groups, which cure by reaction with atmospheric moisture, such products have limited application because they are invariably solvent-containing, difficult to pigment and unsuitable for situations where a high-build of coating, e.g. 100 microns or more, is required.
For the above reasons, compositions based upon two components, usually polyhydroxy materials and polyisocyanates are preferred. Coatings designed on this basis may contain particulate mineral fillers, pigments and other additives or materials, can be applied as thick films in one application and usually offer improved performance compared with the single component moisture curing products. Because of the vast range of available polyhydroxy compounds (polyols), it is possible to produce a wide range of physical properties in the cured products. Coatings may be designed to be very hard, soft and elastomeric, or any physical character between these.
A problem which arises in making polyurethanes is that the polyisocyanates used react easily with water, for example that contained within the raw materials used, in surfaces to which the polyurethane is to be applied or as atmospheric moisture. As well as reducing the number of isocyanate groups available to react with the polyol, the reaction with water generates carbon dioxide, leading to foaming and incomplete cross-linking. This is particularly damaging in the case of protective films. Steps therefore have to be taken to exclude water from the components, and if the polyurethane is to be applied to a wet surface such as an underground pipe, or damp concrete floor, or in external conditions where surfaces are damp or may encounter rain, unwanted isocyanate reaction will inevitably result.
The inevitability of the reaction between isocyanates and water means that all raw materials for inclusion in conventional compositions must be water-free, or must be dried before or during inclusion and that such compositions are totally unsuitable for use where water is present.
EP-A-0 145 269 discloses two-component compositions which are capable of curing in the presence of water to form polyurethane resins. The compositions comprise highly reactive polyols and solvent-based propolymers with less reactivity than monomeric diisocyanates such as methylene di-p-phenylene isocyanate (MDI) to and toluene isocyanate (TDI). A large excess of water and calcium hydroxide is employed by the compositions to act as a scavenger for carbon dioxide produced. The polyurethane resins formed have limited strength and there are disadvantages, such as flammability and environmental and disposal problems of volatile organic solvents, inherent in the use of solvent-containing materials.